Lessons in elementary chemistry: inorganic and organic / by Sir Henry E. Roscoe.

  • Roscoe, Henry E. (Henry Enfield), 1833-1915.
Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary chemistry: inorganic and organic / by Sir Henry E. Roscoe. Source: Wellcome Collection.

    24/530
    ELEMENTARY CHEMESTUV UCSSON clicniical balance is cm])loyed. Fig. 2 represents one form of chemical balance. It consists of a perforated brass beam (.\a) vibrating about its centre, at which is fixed a triangular knife-edge of agate (c): this rests upon a horizontal agate ])lane attached to the u]jright brass pillar. To each end of the beam the light brass pans (im) are attached, each pan being suspended by an agate plane upon an agate knife- edge fixed on the end of the beam at DD. This mode of rest and support is to render the amount of friction as small as possible, and thus to insure delicacy in the instrument. In order to prevent the agate edges from being spoilt by constant wear on the agate planes, the beams and the ends (dh) are supported by the brass arm (f.k) when the balance is not in use, so that the agate surfaces do not touch : the beam and pans are released when I’cquired by turning the handle i'. 'J'he svdrstance to be weighed is placed in one ]ran, and weights added one by one to the other until the instrument is in equilibrium ; this is ascertained by the long pointer (c) vibrating to an equal distance on each side of the central mark. A balance such as that represented in the figure will turn with of a milligram when loaded with too grams (see p. 24), or will indicate with this load the one-millionth part of the substance weighed. The stability of the balance is insured by the centre of gravity being situated below the jroint or axis of suspension. The sensibilitv of the balance is secured by ha\-ing the beam as light and as long as is possible consistenth' with rigidity, and by having the distance between the centre of gravity and the ])oint t>f suspension as small as possible, also b)’ reducing the friction on all the parts to a minimum. The accuracy of the balance depends upon ha^■ing the two arms of equal length. .YU delicate chemical balances arc kept in glass cases to avoid draughts of air, which would disturb the accuiac) of the weighings, as well as to protect the instrument against dust and moisture. The aim of the chemist is to examine the properties of .ill substances with regard to their actions iqron one another in producing bodies essentially diftcring from the originals. In order thoroughly to carry out his purpose he is obliged to resort to experiment; that is, he has to place the substances which he is c.xamining under circumstances, perhaps not